JOURNAL O F T H E A M E R I C A N C H E M I C A L SOCIETY Registered
in
Li. S. Patent Ofice.
a Copyright
1973 b y t h e American Chemical Societ?
OCTOBER31, 1973
VOLUME95, NUMBER22
T h e Radiation Chemistry of Aqueous Solutions of Simple R C N Compounds I. D r a g a d , * Z. Draganib, Lj. Petkovic, and A. Nikolib Contribution f r o m the Boris Kidrir Institute of Nuclear Sciences, P.O. Box 522, 11001 Beograd, Yugoslavia. Receiced April 4 , 1973
The y radiolysis of the following R C N compounds in aqueous solutions was studied at low doses ( < l o krads): hydrogen cyanide, acetonitrile, propionitrile, malononitrile, and succinonitrile. The radiolytic products H z , H?Oz, N H 3 , aldehydes, C 0 2 , and H N O Z were determined in degassed or oxygenated solutions at pH’s 2 and 6. The cyano group was found to be the main target of attack by the primary free-radical products of irradiated water. Its behavior was considered by taking into account the free-radical model of water radiolysis and the radiation chemical yields measured under various experimental conditions. The following rate constants were determined H C N ) = 6.6 X loa M-l sec-l, k(OH in competition experiments: k(H + H C N ) = 3.6 X lo7 M-l sec-l, k(e,,H C N ) 5 5 X lo7 M-l sec-l; k(H + C H 3 C N ) = 3.5 x lo6 M-l sec-’, k(eeqCH,CN) = 2.5 X 10’ M-’ sec-I, k(OH C H 3 C N ) = 5.5 x 106 M-l sec- l ; k(H CzH,CN) = 1.0 X lo7 M-l sec-l, k(e,,C2H5CN)= 1.0 X lo8 M-I sec-l, k(OH + CzH,CN) = 7.3 x lo7 M-1 sec-l; k(H + CH,(CN),) = 1.6 X 10’ M-’ sec-l, &eeq- CH?(CN)J = 7.1 x 103 M-1 sec-1; k ( H (CH,CN)?) = 1.3 X lo7 M-l sec-l, &e,,- i- (CH?CN)*) = 1.7 X lo9 M-l sec-l, k(OH + (CH2CN)?) = 3.0 x 10’ M-1 sec-l. The correlation between Taft’s u* function and measured rate constants of eeq- + R C N reactions shows that the effect of the substituent R is mainly of the inductive type. Abstract:
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T
he radiation chemistry of aqueous solutions of R C N molecules is little known, although the versatility of the cyano group was demonstrated in experiments where the energy was supplied by radiation.’-’ The C N group might have made its appearance in the early stages of the chemical evolution,’,* and this explains the interest in chemical changes in aqueous solutions of R C N compounds in prebiotic and radiation studies. In this work we have investigated the behavior of some simple R C N molecules in dilute aqueous solutions exposed to low doses (< 10 krads) of ionizing radiations (the cobalt-60 y-rays). When the dilute aqueous solutions are irradiated the energy is practically deposited in water only. This leads to the formation of the hydrogen atom, hydrated electron, and hydroxyl radical in known radiation chemical yields. Their natures and the sequence of events that take place in ( 1 ) M. Calvin, “Chemical Evolution,” Oxford University Press, Oxford, 1969. (2) R. M. Lemon, Chem. Rer., 70.95 (1970). (3) I. G. Draganif, Z. D. DraganiC, and R. A . Holroyd, J . Phys. Chem., 75,608 (1971). (4) H. Ogura, J. Radiat. Res., 8,93 (1967). ( 5 ) H. Ogura, Bull. Chem. SOC.Jup., 41,2871 (1968). (6) D. Behar and R . W. Fessenden, J. Phqs. Chem., 76,3945 (1972). (7) (a) P. Neta and R . W. Fessenden, J . PhJ.s. Chem., 74, 3362 (1970); (b) R. Livingston and H . Zeldes, J. Magn. Resonance, 1,169 (1969).
irradiated water are well established.8 This was not only helpful in formulating the reaction mechanism but also in its testing by correlating the energy input, and the yields of primary free radicals, to the measured yields of stable radiolytic products. The compounds investigated are: H C N , CH3CN, C2H,CN, CH2(CN)2, and (CH2CN)?. It was hoped that the variation of the substituent R in the RCN molecule might contribute to a better understanding of the role of the cyano group in processes occurring in irradiated aqueous solutions. Also the previous work with aqueous solutions of (CN),,3 where a high reactivity of the hydrated electron was found, suggested that the influence of the adjoining group on the reactivity of eeQ- toward C N is worth examining. In order to avoid a larger accumulation of stable radiolytic products, their involvement in the reaction mechanism, and the resulting complication of the reaction scheme, we have carried out the irradiations only at low absorbed doses. An earlier study on the radiolysis of aqueous solutions of hydrogen cyanide4s5concerns the y radiolysis at doses higher than in this work by a factor up to 100. Other published data, relevant (8) I. G. DraganiC and Z. D. DraganiC, “The Radiation Chemistry of Water,” Academic Press, New York, N. Y., 1971.
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71 94 Table I. The Yields of Molecular Products in Deaerated Aqueous Solutions of RCN Determined at Low Radiation Doses
2 Natural
2 -6
0. IO 0.08 0.04 0. I O 0.04 0 .006b 0. IO 0.05 0.10
0.40 0.42 0.38 0.42 0.51 0.45
0.10d
2 -6
0.10 0.05 0.10 0.05
0.77 0.51
HCN 2.61 2.36 2.08 2.46 1.69 1.9
-0.8
1.88 1.78 1.29
-0
U
8
0 8
1 , 1c
CH,CN 1.10 0.88 1 .oo 0.40
0.94 0.88 1.50 0.82
0.83
C?H;CN 1.10 0.80 1.31 1.35
1.32 0.82 1.50 1.50
0.11
